This invention relates in general to composite structures and more particularly to a ribbed composite structure and a process and apparatus for producing such a structure.
Where light weight and high strength are required, composite structures composed of filaments imbedded in resin or other martricies compare favorably with light weight metals such as aluminum and titanium. Indeed composite structures are in many instances superior to metals. For example, where the loading is taken in the direction of the fibers, composite structures of significantly lighter weight have greater strength than equivalent metal structures. Also, they are usually less expensive.
Perhaps the best way of increasing the strength of any structure without significantly increasing its weight is to incorporate ribs into the structure, and where the structure must possess equal strength in all directions the most desirable rib arrangement is the so-called isogrid. In this arrangement the ribs intersect at nodes with the ribs at any one node being arranged at 60.degree. angles. %angles. As a result the intersecting ribs form an array of equilaterial triangles.
The isogrid rib pattern is quite difficult to form in metals since it requires machining and these machining operations are from a practical stand point restricted to flat work. Thus, when an isogrid structure is required in a cylindrical or other tubular configuration, the isogrid pattern is machined into flat metal and then the metal is rolled into the tubular configuration and its ends are joined together along an axial seam. The seam of course detracts from the strength of the overall structure.
The isogrid arrangement has heretofore been equally difficult to form in composite structures, but the problems are of a different nature. If the isogrid structure is to have adequate strength, the filaments must not only be continuous through the nodes, but further must not undergo any significant change in direction. This necessitates that the filaments cross at each node and as a result a substantial build-up of filaments occurs at each node, while excessive spacing exists between the filaments beyond the nodes. These spaces are filled with resin which does not have near the strength of the filaments. In addition holes drilled into the nodes for attachment purposes cut filaments and reduce the strength of the overall structure. Some lay-up procedures result in excessive bending of the filaments at the nodes, and this likewise reduces the strength of the overall structure.